Artificial ground freezing methods have been used for ground improvement work using tunnel boring machine (TBM) for the launch area and the arrival are of TBM shafts, cross passage between tunnels, connection in underground tunnels, and enlargement of TBM tunnel (space). For artificial ground freezing methods applying to such objects, since larger and deeper underground structures are required and therefore extremely large scale of freezing is required, it is necessary to maintain ground freezing work in several months or even several years.
As conventionally known, strength of frozen ground depends on the freezing temperature with negative correlation, that is, the strength increases as the temperature descends. Since water cutoff and pressure-proof properties are desired in said frozen ground, it is necessary to maintain a freezing temperature equal to or less than −10° C. in a predetermined mass of ground (with the size determined by thickness, width, and height) for long periods in order to ensure design strengths.
In a typical artificial ground freezing method, basically, there are steps for placing freeze pipes in the ground, circulating a low-temperature coolant in the freeze pipes, cooling the ground around the pipes, and freezing the ground. In cases that tunnels are connected in underground, it is possible to freeze ground around the tunnel boring machine or tunnel linings by providing freeze pipes inside of steel shells of tunnel boring machines or tunnel lining or by placing freeze pipes on said steel shells or tunnel lining.
In methods of cooling freeze pipes, there are two types, that is, brine type and low-temperature liquefied gas type. In the brine type, antifreeze (brine) such as aqueous solution of calcium chloride is cooled at approximate −30° C. by a freezing device provided on the ground, the antifreeze is circulated in the freeze pipes and then cools the ground. On the other hand, in the low-temperature liquefied gas type, liquid-phase nitrogen having been transported by a tank lorry is supplied to the freeze pipes directly in order to cool the ground, the ground is frozen by vaporization heat of the nitrogen and vaporized nitrogen gas is diffused in the atmosphere. Normally, the low-temperature liquefied gas type is applied in short-term and small-scale underground construction work, a frozen soil typical quantity of which is equal to or less than 200 m3, or in soil sampling surveys.
An artificial ground freezing method in which brine is used is primarily applied in the underground tunnel construction, etc., said method with brine is shown in FIG. 16 with a refrigerator.
Specifically, in FIG. 16, a secondary coolant (brine) is cooled by an evaporator 100A of a refrigerator 100, said the temperature of the secondary coolant rises while flowing in freeze pipes 101 positioned in a ground G (two pipes in FIG. 16) in order to freeze the ground around the pipes.
In a system shown in FIG. 16, a primary coolant (Coolant R404a, etc.) of the refrigerator 100 is vaporized by the heat exchanged with the secondary coolant, and then, the vaporized primary coolant is liquefied by the heat exchanged with water in a condenser 100B. The heat transferred from the primary coolant to water in the condenser 100B is diffused by a cooling tower 100C.
Furthermore, the numeral 102 refers to a coolant circulation pump.
In a prior art brine method shown in FIG. 16, brine is cooled at approximate −30° C. by a freezing device (refrigerator) installed on the ground. In a case that artificial ground freezing method is applied for ground improvement work using tunnel boring machines in the launch areas and the arrival areas of TBM shaft, connecting passage between underground tunnels or underground tunnel connecting areas, there is a problem that large quantity of energy is necessary in order to cool a large amount of brine in low temperature, if a scale of ground size to be frozen is extremely large.
Also, since brine is a highly viscous fluid with the coefficient of viscosity approximate ten times as large as the coefficient of viscosity of water, a diameter of freeze pipes must be large, and also, the brine must circulate in the freeze pipes with high flow rate in order to efficiently absorb heat from the ground to be frozen. For these requirements, freeze pipes with a large diameter are necessary, and therefore, higher expenses for boring and pipe materials are required. Furthermore, higher capacity in brine circulation pump being required leading to higher rental cost of brine circulation pump and larger pump driving energy, and therefore, economically disadvantage generates.
A freeze pipe used in a prior art brine method is mainly a double-pipe structure comprising an outer pipe in which supply-brine flows from a freezing device and an inner pipe in which brine absorbing underground heat flows. Such a freeze pipe may be a steel pipe, e.g., a gas pipe, which must extend and is buried in the ground from a few meters to 100 meters in the vertical direction. Upon production at a plant, steel pipes, each cut into a fixed length of 5.5 m, are transported to a construction site by truck, then, welded above a borehole, at the construction site, and inserted into underground area as freeze pipes.
In a case that a diameter of freeze pipe is large, there is a problem that truck transportation cost and rental cost of a crane for pipe lifting are expensive, large effort for welding is required, and therefore, economically disadvantage is generated. Also, there are other economic problems that a time periods of steps for boring and for placing freeze pipes of underground construction become longer respectively and the entire construction costs are extremely expensive.
Also, if underground brine leakage from defective welded freeze pipes generates, the ground around the leakage position fails to sufficiently freeze, water leakage is generated and insufficient ground strength will not be accomplished, and therefore, it is difficult to carry out subsequent construction work.
On the other hand, in a prior art low-temperature liquefied gas type, liquefied carbon dioxide is injected into the ground and freezes the soil around pipes by heat of vaporization of the liquefied carbon dioxide (Patent Document 1), and in a case said prior art type is applied to ground improvement work using tunnel boring machine in the launch areas and the arrival areas of TBM shaft, connecting passage between underground tunnels or underground tunnel connecting areas, a large amount of liquefied carbon dioxide is necessary to maintain a state that a soil is frozen for long periods.
Here, when the neighboring ground to the injection point of liquefied carbon dioxide starts to be frozen, since it is difficult to deliver liquefied carbon dioxide behind the frozen ground, there is a problem that it is impossible to form frozen soil with a temperature being equal to or less than −10° C.
In a prior art method in which a double-pipe structure is used and ground is frozen by utilizing liquid-phase nitrogen with an extremely low boiling point (Patent Document 2), since nitrogen gas is eventually released as “waste” into the atmosphere, there is an economically disadvantage in a case that a scale of the method being carried out is large and consumption quantity of nitrogen gas is large. In addition, an oxygen concentration at a construction site is descended, when large quantity of nitrogen is discharged into the ground or the atmosphere.